This invention relates to a method for improving the quality of electrolytically-produced chlorine, according to which the chlorine is contacted with a solution containing hydrogen chloride in specified quantity, or containing an equivalent combination of hydrogen and chloride ions. In certain preferred aspects, the invention relates to improvements in established methods for the cooling and drying of an electrolytically-produced crude chlorine vapor.
The electrolysis of common salt brine in particularly-designed electrolytic cells is estimated to account for well over ninety percent of world-wide commercial chlorine production. As an electric current is passed through the typical cell, it functions to convert sodium chloride and water into three primary products--chlorine, hydrogen, and caustic soda (sodium hydroxide solution).
It is generally known that the crude chlorine vapor product of brine electrolysis contains a number of impurities. Certain of the impurities arise from the presence of contaminants such as sodium carbonate, ammonia, and air in the brine feed to the cells. Others, for example, oxygen and hypochlorite and perchlorate compounds, are known to result from side reactions in the cells. Carbon dioxide and various organic chlorides are commonly formed in cells equipped with graphite anodes. Still other impurities, e.g., entrained brine mist and hydrogen, are carried into the chlorine as a result of imperfect separation of product and feed streams by the cells. Substantial quantities of water vapor are always found in the crude chlorine stream.
It is now general practice in the industry to subject the hot, wet chlorine vapor flowing from the electrolytic cells to cooling, drying and, optionally, liquefaction procedures, each of which also accomplishes some degree of chlorine purification. Under these procedures, which are so commonly employed as to be considered the industry standard, crude chlorine from the cells is first cooled from about 95.degree. C. to about 15.degree. C., either by direct contact of the crude chlorine vapor with water, or through surface heat exchange of the chlorine or through a combination of direct water contact and surface exchange. Most of the water vapor and brine spray are thereby removed. To yield a dry finished product the cooled gaseous chlorine is then passed in countercurrent contact with concentrated sulfuric acid to accomplish removal of essentially all of the remaining water vapor and brine entrainment. Finally, the chlorine gas may, if desired, be compressed and condensed to yield a liquid product. To a large extent, the more volatile impurities, e.g., hydrogen, oxygen, nitrogen, and carbon dioxide, remain in the vapor state following liquefaction and are thus separated from the finished liquid chlorine.
The principal use of electrolytically-produced chlorine is in the chemical industry, where it is employed as an important reactant in organic and inorganic synthesis processes. In the course of an investigation relating to the use of chlorine for such purposes, it has been found that one or more impurities, which are present in chlorine produced through the electrolysis of brine and which are not effectively removed by treatment of the chlorine according to common cooling, drying and liquefaction procedures, have a notable adverse affect upon the performance of the chlorine in certain organic synthesis reactions. Although these one or more impurities, which have not been conclusively identified, are believed to be present in the chlorine only in very small quantity, i.e., on the order of 5 to 300 ppm, they have been found to have a dramatic effect upon the selectivity in the reactions of certain organic species with chlorine to yield desired reaction products. The primary object of this invention is improvement of the quality of electrolytically-produced chlorine through the effective removal of these one or more impurities.
It has been stated that the invention generally relates to a process in which electrolytically-produced chlorine is contacted with liquid solutions comprising small quantities of hydrogen chloride. It should now also be noted in this regard that hydrogen chloride (HCl) is conventionally utilized for a number of purposes in the course of conventional electrolytic chlorine manufacture from brine. For instance, HCl is generally added to the brine feed to the cells for pH control. In addition HCl is often added to the spent brine from the cells to convert the hypochlorous acid dissolved therein into a recoverable chlorine gas. Of specific interest with regard to the present invention is the role of HCl in certain specialized prior art processes for chlorine purification. For instance, U.S. Pat. No. 3,568,409 discloses that nitrogen trichloride, water, and brine spray can be effectively removed from chlorine through contact with hydrochloric acid having a critical concentration of between twenty and thirty percent by weight. German Pat. No. 878,196 describes a process for water removal from chlorine which involves contact with concentrated hydrochloric acid at a temperature below 0.degree. C. Both of these prior art processes employ solutions having high concentrations of hydrogen chloride expressly for purposes of water removal from chlorine. A dilute aqueous hydrogen chloride solution, as is utilized in certain aspect of the process of the invention, would be detrimental to the accomplishment of this object. No prior art is known to disclose or suggest the purification of electrolytically-produced chlorine through its contact with solutions comprising low concentrations of hydrogen chloride.